Which of the elements Na, Mg, Al and Si has the highest second ionisation enthalpy?

After losing the first electron, $\mathrm{Na^+}$ has the stable neon configuration $2s^22p^6$; removing a second electron means breaking into a filled inner shell with high effective nuclear charge, costing about $4560\ \mathrm{kJ\,mol^{-1}}$. For Mg, Al and Si the second electron comes from the outer shell of the $\mathrm{M^+}$ ion ($\approx1450$, $1820$, $1580\ \mathrm{kJ\,mol^{-1}}$ respectively). So Na has by far the largest IE$_2$.